The present invention relates to intrusion detectors and in particular relates to a new arrangement and method for processing the signals received from sensors used in intrusion detection systems.
Passive infrared intrusion detectors, microwave detectors and ultrasonic detectors are often used in paired combinations to provide a system having a dual technology which is less prone to false alarms and is generally considered more reliable. The combination of passive infrared and microwave detectors is quite common, as the type of situations which can cause false alarms are generally not common to each detector, thus reducing the likelihood of false alarms. The combining of different detectors improves reliability and increases sophistication.
A number of existing dual technology intrusion detection systems make an evaluation of whether the overall system is working satisfactorily or whether the system, although not producing false alarms, may be in trouble. One such assessment of trouble is derived from counting the number of times one of the sensors produces an alarm output which is unconfirmed by the other detector. Typically there is some sort of decay function to decrease the number of false alarms counted at a certain rate, however, should the number of false alarms reach a predetermined maximum, a trouble indication is generated. Other dual technology systems look at the difference in numbers between the false alarms of each sensor for a further evaluation of whether the overall system is working satisfactorily. The generation of false or unconfirmed alarms as an assessment whether the overall system is working satisfactorily has the disadvantage in that a large portion of the information contained within the signal from the sensor is not evaluated except to confirm when the signal has exceeded the alarm threshold condition. This assessment of trouble is also governed by the alarm criteria, which may not be the best assessment of whether the system is operating satisfactorily or operating within a satisfactory environment.
The signals from the different type of sensors are well known and are analysed with respect to particular criteria to derive a signal which is appropriately processed to determine whether an alarm condition exists.
The prior art systems have focused on alarm criteria and have included various compromises made to allow the two technologies to effectively monitor the same area. These compromises must take into account different operating environments and to reduce the possibility of false alarms. A detection system which produces false alarms is most troublesome and the industry is striving to produce systems which do not produce false alarms. Therefore, the industry is faced with the dilemma of trying to reduce false alarms while also providing a system which produces an alarm when an intruder enters the monitored space.
The signal from a passive infrared detector with respect to the disturbances which occur in the area being monitored can be characterized as an alternating signal sometimes considered predominantly sinusoidal whose magnitude typically varies between 0 and 3.6 volts peak to peak (5 volts supply) and whose frequency varies from 0.1 to 10 hertz.
Some approaches for analysing this signal from the passive infrared detector include the use of two comparators, one for evaluating positive portion of the signal and the other for evaluating the negative portion. Pulses are produced when the signal exceeds the threshold the respective comparator and are of a duration corresponding to the time that the signal remains above the minimum threshold. Thus positive pulses of variable duration have been derived by use of two comparators for evaluating positive and negative portions of the signal from the infrared detector. It is also possible to rectify the signal and merely use a single comparator for evaluation of the signal. The problems with the comparator approach is that it is difficult to determine what the best minimum threshold is. A number of factors can affect the signal from the detector and not all of these disturbances indicate that a burglar or intruder is present. RF transient signals produced when switching walkie-talkies between a receive and transmit mode, or the like RF transient signals, can produce a very strong, short duration signal. Heaters coming on within the monitored area can produce a detectable signal, as well as small animals such as a cat, etc., crossing through the zone. Therefore, a problem arises in trying to distinguish between the presence of a human intruder and a disturbance in the signal which is not produced by such an intruder. Use of this alarm criteria includes many compromises and much of the signal from the detector is ignored (i.e. all of the signal below and above the threshold).
A different approach has been to integrate the output signal to provide a measurement of the energy of the signal and it is believed this measurement is more indicative of whether an intruder is present. Unfortunately other factors enter into the consideration such as the ability of the system to detect the desired intruder at a long distance from the detector which typically produces a fairly low frequency signal. Other problems also occur due to the widely varying ambient temperature conditions that can occur in the monitored area. Analysis of the whole signal fails to recognize the different signals which can be partially evaluated by amplitude evaluation alone or in combination with duration and/or shape evaluation.
Many systems have used a single comparator to produce a pulse which is counted, and if sufficient pulses are produced within a certain time period an alarm condition is produced. Counting arrangements can produce false alarms as common environmental disturbances such as blasts of hot air from the heating vents will produce the same unit of information as the sensing of a valid target. In order to reduce the occurrence of false alarms it is possible to increase the comparator trip threshold and/or increase the number of pulses counted before an alarm is generated. Both of these techniques will indeed improve the false alarm immunity however this will be accomplished at the expense of the detection range of the unit. If the number of pulses counted before an alarm condition is produced is increased far detection range will be decreased since far targets will produce few pulses (due to low amplitude and frequency). If the thresholds are increased, far response will again be reduced since the far signals are of lower amplitude. It is for these reasons that maximum pulse setting allowed is typically 3.
In one prior art arrangement the output signal from the detector is fed into an absolute value circuit and subsequently to a voltage controlled pulse generator subsection. When the signal reaches a minimum amplitude the voltage controlled pulse generator begins to produce constant width pulses at a repetition rate proportional to the amplitude of the signal typically in the hundreds of hertz. These pulses are counted or integrated and stored by the means of a capacitor. When the stored energy reaches a preset level an alarm signal is generated. This system suffers the same basic draw backs as a window comparator system in that slowly changing low amplitude transients which barely cross over the threshold generate full amplitude pulses which are integrated towards a possible alarm generation.
Since the slow transients are allowed to produce the same unit of information as valid distance targets, the low frequency response of the amplifier has been set to de-emphasize low frequency response to reduce the probability of false alarms. Unfortunately since distant valid targets produce low frequency signals the overall pattern coverage is decreased as a result.
According to a different arrangement the sinusoidal signal is fed into an absolute value circuit and when this signal exceeds a minimum threshold its amplitude is used to vary the charge current of a capacitor which is used as a energy storage device. The charging current equation is EQU I.sub.charge =(V.sub.signal -V.sub.minimum threshold)/R.sub.charge
When a certain amount of energy over time (in volt seconds) has been accumulated in the capacitor the unit will signal an alarm. This technique is an improvement over previous methods in that the effects of low amplitude transients which barely cross over the minimum threshold are reduced. This is accomplished as their energy over time is low and thus their contribution to the accumulated total energy is low. This technique does require the gain of the amplifier to be excessively high to quickly generate an alarm condition by far-off targets moving at low speed. This presents a problem for RF induced transients which are greatly amplified as a result of this excessive gain requirement.
The present invention seeks to overcome the problems associated with the prior art techniques and provide a system having improved information processing allowing more accurate evaluation of the signal. The invention in the simplest form is relatively inexpensive but the system is also capable of a high degree of sophistication and evaluation of the signal for more demanding applications. The invention recognizes that the alarm criteria is not necessarily the most appropriate to determine a trouble signal indicative of changes in the working environment or whether the environment is such that the alarm criteria must be changed or the overall operation of the system reassessed.